Acoustic Thevenin calibration is a method used to determine the equivalent Thevenin parameters of acoustic probes used in hearing diagnostics and hearing aids (HA). Thevenin calibration is an important calibration step to perform when measuring acoustics in for example hearing diagnostic applications, this being due to the high accuracy requirements needed when performing hearing diagnostics to evaluate a potential hearing loss. Therefore, the acoustic probes used in at least hearing diagnostic applications should be calibrated prior to the actual diagnostic measurements.
Acoustic Thevenin calibration determines the source characteristics (i.e. the source pressure and the source impedance) of the acoustic probe to be used for measurements in an object, such as the ear canal of a test-person. Finding the source characteristics of the acoustic probe from a calibration step, makes it possible to measure any load impedance applied to the probe or HA. Thus, a Thevenin calibration of an acoustic probe is a calibration step, which is usually performed prior to the actual measurements in an object, such as in the ear canal of a user, for which the impedance should be measured for the purpose of e.g. providing a diagnosis. Similarly, within other acoustic applications, such as musical acoustics, a Thevenin calibration may also be applied to the acoustical probes prior to the measurement of e.g. impedance of an acoustic instrument, such as a musical instrument, to obtain a performance characteristic of the acoustic instrument.
Accordingly, it is generally appreciated within the field of measuring the acoustic characteristics of an object to perform a Thevenin calibration of acoustic probes prior to the actual measurements of an object of interest.
The Thevenin calibration method is based on presenting a number of reference loads to the probe or HA, whose impedances are known or can be calculated analytically. Typically, these loads are hard walled cylindrical waveguides of different lengths. The response in each waveguide is then used to find the Thevenin parameters (i.e. the source characteristics of the probe) using a least squares fit (i.e. solving the least square fit equation to find the source pressure and the source impedance).
This calibration procedure is in practice very sensitive to the analytical or assumed plane wave impedance of the waveguide. In reality, the true impedance (i.e. the load impedance measured) as seen by the probe (or HA), differs from the analytical impedance due to phenomena associated with the sound transitioning from a narrow delivery orifice (such as a tube or an annular slit) in the probe (or HA) to a wider waveguide. The sound pressure is measured in close proximity to the delivery tube, introducing an error in the measured frequency response function.
Within the field of acoustic applications, it is generally known that such errors introduced are caused by a geometrical mismatch between the acoustic probe and the load applied thereto, and which are at least related to evanescent modes. Thus, there has been a general need to avoid the effects of at least evanescent modes during impedance measurements of an object of interest.
One approach used for avoiding evanescent modes when measuring the acoustic impedance of a real ear set-up (i.e. an acoustic probe tube being inserted into the ear canal of a test-subject) has been focused on sufficiently attenuating any localized, non-propagating acoustic field caused by evanescent modes. This has been achieved by restricting the frequency content of the external stimuli or by drawing the probe microphone, recording the response in the test-object, slightly beyond the plane of the probe transducer emitting the sound stimuli into the test-object. In other words, one method to compensate for evanescent modes is by protruding the measurement microphone of the acoustic probe a given distance beyond the plane of the probe tip, whereby the probe response is significantly less affected by evanescent modes. Main drawback with this approach is that the excess waveguide between the source outlet and microphone inlet is included in the source characteristics rendering the calibration invalid when inserted into a waveguide of different dimensions.
Other methods focusing on compensating impedance measurements for the errors introduced by a geometrical mismatch are aiming at applying a correction factor to the resulting impedance measurements performed on a test-object and subsequent to any probe calibration procedure.
As previously elaborated on, it is known within the art to perform acoustic input impedance measurements of acoustic waveguides using a traditional impedance probe comprising an annular sound emitting slit assumed to provide a constant volume velocity. Furthermore, the acoustic probes used to measure the input impedances are, as already explained, in a first step, calibrated prior to the real impedance measurements, so as to obtain the Thevenin parameters (i.e. source characteristics) of the acoustic probe used for the impedance measurements. In a second subsequent step, the calibrated acoustic probe is inserted into the test-object, device, another waveguide or instrument, of which the impedance should be measured. As previously described real impedance measurements, whether being performed in e.g. an ear canal or a musical application, may also experience errors related to geometrical mismatch between the acoustic probe and the test-object.
In addition to the already described prior art method, other suggested methods for compensating for such errors in the measured impedance subsequently to calibration of the acoustic probe used for the measurement, therefore includes a correction of the obtained impedance measurement results. Such corrections are suggested to be made by visual inspection of the measured impedance, and includes an imaginary, frequency proportional correction factor, which is adjusted such that impedance minima are placed half-way between two subsequent maxima in the impedance measurements. Furthermore, a real correction factor proportional to the square root of frequency is adjusted such that the envelope of impedance minima is equal to the envelope of impedance maxima. This method of correcting for a geometrical mismatch entails some constraints to the subsequent measurement with the acoustic probes. When finding the correction factors of the calibrated acoustic probe, this is done in a tube having a specific geometry matching that of the object to be measured. This entails the constraint that the measurement device to be measured in a final impedance measurement should substantially be coupled to the acoustic probe through tubes having the same diameter as the tube which was used for estimating the correction factors. Thus, the correction factors related to the geometrical mismatch found by this method can only be used on a limited number of actual devices having substantial the same geometry as the one assumed during the correction measurements in front of the probe.
As is apparent, the known methods are not related to determining the source characteristics of the probe, which characteristics are determined in a previous calibration step in a traditional way by placing the annular probe in a semi-infinite waveguide. The calibration is possible using only a single load since this probe is assumed a constant volume velocity source. This is equivalent to assuming an infinite source impedance in the Thevenin parameters. Such calibration will be affected by evanescent modes, but since no impedance minima are present in the impedance spectrum of a non-reflecting load the relative influence will be negligible.
Furthermore, the known methods are suggesting compensating for evanescent modes subsequent to the calibration of the acoustic probes which should be used for impedance measurements, however entailing some limitations to the subsequent actual device measurements. Thus, potential errors introduced already in the calibration of the acoustic probe are not accounted for in current methods.
Therefore, there is a need to provide a solution that removes or at least reduces the calibration errors associated with the above mentioned geometric mismatch between the probe and the given waveguide.